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A) \(\frac{7}{\left(x+3\right)\left(x+10\right)}+\frac{11}{\left(x+10\right)\left(x+21\right)}+\frac{13}{\left(x+21\right)\left(x+34\right)}\)
\(=\frac{\left(x+10\right)-\left(x+3\right)}{\left(x+3\right)\left(x+10\right)}+\frac{\left(x+21\right)-\left(x+10\right)}{\left(x+10\right)\left(x+21\right)}+\frac{\left(x+34\right)-\left(x+21\right)}{\left(x+21\right)\left(x+34\right)}\)
\(=\frac{1}{x+3}-\frac{1}{x+10}+\frac{1}{x+10}-\frac{1}{x+21}+\frac{1}{x+21}-\frac{1}{x+34}\)
\(=\frac{1}{x+3}-\frac{1}{x+34}\)
\(=\frac{\left(x+34\right)-\left(x+3\right)}{\left(x+3\right)\left(x+34\right)}\)\(=\frac{x}{\left(x+3\right)\left(x+34\right)}\)
\(\Rightarrow\left(x+34\right)-\left(x+3\right)=x\)
\(\Rightarrow x=31\)
Vậy, x = 31
Bạn áp dụng: \(\frac{k}{x\cdot\left(x+k\right)}=\frac{1}{x}-\frac{1}{x+k}\) với \(x,k\inℝ;x\ne0;x\ne-k\)
Chứng minh: \(\frac{1}{x}-\frac{1}{x+k}=\frac{x+k}{x\left(x+k\right)}-\frac{x}{x\left(x+k\right)}=\frac{x+k-x}{x\left(x+k\right)}=\frac{k}{x\left(x+k\right)}\)
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Đặt \(\frac{x_1-1}{9}=\frac{x_2-2}{8}=.....=\frac{x_8-8}{2}=\frac{x_9-9}{1}=k\)
Áp dụng TC DTSBN ta có :
\(k=\frac{\left(x_1-1\right)+\left(x_2-2\right)+...+\left(x_8-8\right)+\left(x_9-9\right)}{9+8+....+2+1}\)
\(=\frac{\left(x_1+x_2+....+x_9\right)-\left(1+2+....+8+9\right)}{1+2+3+...+8+9}=\frac{900-45}{45}=19\)
\(\Rightarrow\frac{x_1-1}{9}=\frac{x_2-2}{8}=.....=\frac{x_8-8}{2}=\frac{x_9-9}{1}=19\)
\(\Rightarrow x_1=172;x_2=154;x_3=136;x_4=118;x_5=100;x_6=82;x_7=64;x_8=46;x_9=18\)
Áp dụng t/c dãy tỉ số bằng nhau ta có :
\(\frac{x_1-1}{9}=\frac{x_2-2}{8}=...=\frac{x_9-9}{1}=\frac{x_1-1+x_2-2+...+x_9-9}{9+8+...+1}\)
\(=\frac{\left[x_1+x_2+...+x_9\right]-\left[1+2+3+...+9\right]}{9+8+...+1}=\frac{900-45}{45}=19\)
Ta có : \(\frac{x_1-1}{9}=19\)=> \(x_1-1=171\)=> \(x_1=172\)
Từ đó ta tìm được : x2 = 154 , x3 = 136 , x4 = 118 , x5 = 100 , ...
Đến đây tìm được các x còn lại
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6^15*9^10/3^34*2^13
=2^15*3^15*3^20/3^34*2^13
Rút gọn phân số trên, ta được
2^2*3/1=12
Vậy phép tính trên bằng 12
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a) \(\left|\frac{-15}{6}\right|-\left|\frac{3}{18}\right|.\sqrt{81}+\sqrt{\frac{9}{64}}\)
\(=\frac{15}{6}-\frac{1}{6}.9+\frac{3}{8}\)
\(=\frac{15}{6}-\frac{9}{6}+\frac{3}{8}\)
\(=1+\frac{3}{8}\)
\(=\frac{11}{8}\)
b) \(\frac{6^{15}.9^{10}}{3^{34}.2^{13}}=\frac{\left(2.3\right)^{15}.\left(3^2\right)^{10}}{3^{34}.2^{13}}=\frac{2^{15}.3^{15}.3^{20}}{3^{34}.2^{13}}=2^2.3=12\)
a/ \(\left|\frac{-15}{6}\right|-\left|\frac{3}{18}\right|.\sqrt{81}+\sqrt{\frac{9}{64}}\)
= \(\frac{15}{6}-\frac{3}{18}.9+\frac{8}{8}\)
= \(\frac{15}{6}-\frac{3}{2}+\frac{3}{8}\)
= \(\frac{60-36+9}{24}=\frac{33}{24}=\frac{11}{8}\)
b/ \(\frac{6^{15}.9^{10}}{3^{34}.2^{13}}=\frac{\left(2.3\right)^{15}.\left(3^2\right)^{10}}{3^{34}.2^{13}}\) \(=\frac{2^{15}.3^{15}.3^{20}}{3^{34}.2^{13}}=\frac{2^2.3^{35}}{3^{34}}=\frac{4.3}{1}=12\)
\(\frac{6^5.9^{10}}{3^{34}.2^{13}}\)
\(=\frac{\left(2.3\right)^5.\left(3^2\right)^{10}}{3^{34}.2^{13}}\)
\(=\frac{2^5.3^5.3^{20}}{3^{34}.2^{13}}\)
\(=\frac{2^5.3^{25}}{3^{34}.2^{13}}\)
\(=\frac{1}{3^9.2^8}\)